(1) Field of the Invention
The present invention relates to heavy duty pneumatic radial tires for use in heavy vehicles such as trucks and buses, having tread patterns which are required for all-weather type tires for continuously travelling very long distances and which effectively reduce various kinds of uneven wear which is apt to occur in such tread patterned tires.
(2) Description of the Prior Art
In general, the heavy duty pneumatic radial tires having stiff reinforced layers such as metal cord layers are superior to the conventional bias tires in resistance to wear of treads, puncture and the like. In consequence of the remarkable improvement of highways, the demand for the heavy duty radial tires has been rapidly increased. On the other hand the performance of vehicles has been improved, so that travelling distances of vehicles or cars have been considerably spread in conjunction with the development of the highways. Accordingly, a new performance other than the above has been needed for these tires of this kind.
The new requirement for the tires is to have an all-weather property, that is, a performance capable of continuous travelling for a long period of time without being affected by all the conditions on roads (snowing, freezing, raining, flood, etc.) encountered during travelling long distances.
Performance of this kind generally depends upon the tread pattern. Block type tread patterns have been mainly developed for this purpose.
For the above purpose, however, there is a great difficulty with tires having block type tread patterns hitherto used, resulting from the fact that the continuous use of the tires for a long period of time causes extraordinarily irregular wear (ununiform or uneven wears) on the treads to shorten the life of the tires, because the tread patterns cannot endure severe conditions when travelling long distances for a long time, although they have been developed to fulfil the requirements for variations in weather conditions.
The above ununiform or uneven wears are generally classified into the following three kinds:
(1) Shoulders tend to wear more rapidly than crown centers of treads. Such a wear is referred to as "shoulder wear" and indicated by W.sub.s hereinafter.
(2) Rear edges of blocks in the patterns tend to wear to a greater extent than front edges which are brought into contact with a road surface earlier than the rear edges, to thus produce uneven wear of the blocks in the patterns. Such a wear is referred to as "heel-and-tow wear" and indicated by W.sub.h.
(3) Edges of ribs or blocks in opposition to grooves extending in a circumferential direction of the tire tend to wear to form depressions along the grooves. This wear is referred to as "railway wear" and indicated by W.sub.r.